Chemical Science View Article Online PERSPECTIVE View Journal | View Issue Challenges and prospects of ambient hybrid solar cell applications Cite this: Chem. Sci.,2021,12, 5002 ab b ab All publication charges for this article Hannes Michaels, † Iacopo Benesperi† and Marina Freitag * have been paid for by the Royal Society of Chemistry The impending implementation of billions of Internet of Things and wireless sensor network devices has the potential to be the next digital revolution, if energy consumption and sustainability constraints can be overcome. Ambient photovoltaics provide vast universal energy that can be used to realise near- perpetual intelligent IoT devices which can directly transform diffused light energy into computational inferences based on artificial neural networks and machine learning. At the same time, a new Received 25th November 2020 architecture and energy model needs to be developed for IoT devices to optimize their ability to sense, Accepted 3rd March 2021 interact, and anticipate. We address the state-of-the-art materials for indoor photovoltaics, with DOI: 10.1039/d0sc06477g a particular focus on dye-sensitized solar cells, and their effect on the architecture of next generation rsc.li/chemical-science IoT devices and sensor networks. Creative Commons Attribution 3.0 Unported Licence. Introduction including that in low light and indoor environments. Even if the available energy under such circumstances is 100–1000 times Light in itself is the only form of energy visible to the human lower compared to that of direct solar illumination, it can still eye. Photovoltaic devices, such as solar cells, are capable of serve the critical energy required for low-power applications, converting light into electricity. They represent a sustainable including the rapidly growing family of the Internet of Things power source, which will play a major role in the quest to (IoT). decarbonize human energy production. The continuous inno- As billions of wireless sensors are to be introduced over the vative research towards more efficient and sustainable solar coming decade – with about half of them to be situated indoors This article is licensed under a cells opens the possibility to utilize all available light energy, – a pending question was posed by Hittinger and Jaramillo: if the IoT evolution will actually yield energy savings, given that said sensors have to be powered rst (Fig. 1).1 Currently, the a Open Access Article. Published on 09 March 2021. Downloaded 9/27/2021 5:32:14 AM. Department of Chemistry, Angstr˚ om¨ Laboratory, Uppsala University, P.O. Box 523, major constraining factor for IoT devices is their reliance on SE-75120 Uppsala, Sweden battery power or grid wired connections. As such, sacricing b School of Natural and Environmental Science, Newcastle University, Bedson Building, performance for increased battery life and recurring mainte- NE1 7RU Newcastle upon Tyne, UK. E-mail: [email protected] nance needs severely impact their suitability and sustainability. † These authors contributed equally to this work. Hannes Michaels obtained his Dr Iacopo Benesperi received his BSc degree from the University of MSc in Materials Science from Hamburg, Germany, in Nano- the Department of Chemistry, science in 2015 and his MSc Turin University, Italy, under degree in 2017, aer contribu- the super-vision of Prof Barolo. tions to the research groups of During his PhD in Chemistry at Prof. Dr Hans-P. Oepen and Monash University, Melbourne, Prof. Dr Wilfried Wurth. Since Australia, he worked on hole 2017, he is a doctoral student at conducting materials for perov- the Department of Chemistry in skite solar cells under the Uppsala, Sweden, exploring supervision of the late Prof. charge transport materials Spiccia, Dr Simonov, Prof. Bach based on transition metal and Prof. Cheng. He is currently complexes for photovoltaic applications in the laboratory of Dr a research fellow at Uppsala University in Dr Freitag's group, Marina Freitag. working on copper complex charge transport materials for thin- lm solar cells. 5002 | Chem. Sci.,2021,12,5002–5015 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Perspective Chemical Science Fig. 1 The Internet of Things can connect sensors deployed remotely, but these sensors must be powered. (a) Photovoltaics could power sensors both inside buildings and in the environment, as shown in this example of a “smart ranch”. Reproduced from Haight et al., ref. 3, with permission from Science AAAS, copyright 2016; (b) example of a 8 cm  1 cm dye-sensitized solar cell; (c) operational principle of dye-sensitized solar cells. Reproduced from Jiang et al., ref. 32, with permission from Elsevier, copyright 2019. Creative Commons Attribution 3.0 Unported Licence. Powering the IoT with light harvesters instead has complex larger bandgap of around 1.7 eV – introduces deciencies in effects on energy use. Integrating indoor photovoltaics (IPVs) charge transport.4 III–V materials such as GaAs and its deriva- into IoT devices will render the future wireless networks and tives (CIGS) match indoor illumination better in terms of sensors autonomous, providing greater reliability and opera- absorption prole compared to silicon and up to 28% conver- tional lifetimes. sion efficiency have been achieved.5,6 Generally, however, CIGS Ruhle¨ et al. calculated that the maximum theoretical cells are fabricated at high cost and will remain constrained to conversion efficiency for IPVs – as an analogue of the famous niche applications.7 Shockley–Queisser limit of 33.7% – could surpass 52%, owing to Third generation photovoltaics – including organic (OPVs) the narrower spectral distribution and thus reduced thermal and hybrid solar cells (perovskite, PSCs and dye-sensitized, This article is licensed under a losses.2,3 The narrow bandgap of crystalline silicon (1.1 eV), as DSCs) – are contenders for commercialization, as their used in most outdoor solar cells, leads to high thermal losses absorption properties and architectures can easily be adapted to above near-infrared radiation, making this technology imprac- ambient light conditions. In 2018, Lee et al. reported an organic Open Access Article. Published on 09 March 2021. Downloaded 9/27/2021 5:32:14 AM. tical indoors. Employing amorphous silicon instead – given its solar cell with 28% conversion efficiency under ambient conditions.8 DSCs are the technology of choice for indoor applications, with good shunt blocking properties, high pho- tovoltages and great adaptability. Their modularity allows the Dr Marina Freitag is presently choice of different dyes to match a variety of light sources, a Royal Society University making DSCs a unique choice for most indoor settings, with Research Fellow and Newcastle efficiencies up to 34%.9–11 The only other photovoltaic tech- Academic Track Fellow at School nology that, like DSCs, has reached efficiencies beyond 30%, is of Natural and Environmental that of perovskite solar cells, with recent ndings reporting Sciences, Newcastle University, photovoltages exceeding 1 V at 1000 lx illumination.12–14 None- UK. She is continuing her work in theless, and especially for indoor applications, lead perovskite coordination chemistry, energy solar cells are severely limited by their toxicity. Prominent relevant materials and solar examples of sectors that will benet from photovoltaic-powered cells. Previously, she held as an sensors comprise agriculture, healthcare and education.15–17 In Assistant Professor at the all the aforementioned areas, IPVs will be in much closer Department of Chemistry, proximity to humans compared to large-scale outdoor solar Uppsala University and a post- installations. As such, toxicity concerns of lead perovskites as doc position at EPFL (Prof. Anders Hagfeldt's). She received her analyzed by Babayigit et al. become of even higher importance.18 PhD degree with Prof. Galoppini from Rutgers University, USA, and Attempts to replace lead in halide perovskites with its lighter her BSc, in Chemistry from Freie Universitat¨ Berlin, Germany. She homologues has resulted in materials with smaller bandgaps, is the inventor of the so-called Zombie-cell and she contributed to thus less suited for indoor photovoltaic cells, and only doubt- recent breakthroughs in dye-sensitized solar cells for low-power fully reduced toxicity.19 The main concern for the applications. © 2021 The Author(s). Published by the Royal Society of Chemistry Chem. Sci.,2021,12,5002–5015 | 5003 View Article Online Chemical Science Perspective commercialization of third generation photovoltaics is that of long-term stability. While increasingly stable photovoltaic performances can be found in the literature for OPVs, PSCs and DSCs,20–23 so far only a DSC study has directly tracked the output power of a solar cell array under load in an ambient setting over weeks.9 Comprehensive comparisons of different types of indoor photovoltaics can be found in the literature.24–31 To date, there has been no general agreement on how IPV technologies should be compared and improved. Therefore, we will address this aspect here. Other pressing issues are related to materials development for higher sustainability and effi- ciency as well as hardware advancement for low energy elec- tronics and energy-efficient machine learning capabilities. The most efficient way of operation is to convert ambient light directly to structured information in one system. At the end of the day, the IoT can only become the next technological revo- lution if the right symbiosis between energy efficiency and Fig. 2 Normalized emission spectra of warm white CF and LED bulbs, sustainability is found. and of the AM1.5G standard. Towards a consensus in term, and all are currently employed in different settings. As standardization such, it is difficult to create a standard for indoor illumination. Moreover, light bulb technology progresses every year and Creative Commons Attribution 3.0 Unported Licence. Accurate and reproducible measurements are key in the elds indoor measurements should follow its advances to provide of Science and Engineering. Concerning photovoltaics, an results that are close to real operational cases.